The present invention relates to a permanent magnet type electric motor and, particularly, to a structure thereof suitable for a smooth arrangement of through-bolts between field magnetic poles mounted on an inner surface of a cylindrical yoke of the motor.
FIGS. 1 and 2 show a conventional d.c. motor of this type, in which the motor is composed of a cylindrical yoke 1 on an inner peripheral surface of which a plurality of field magnetic poles 4 each composed of a permanent magnet 2 and an auxiliary pole 3 are arranged in parallel with a space 5 between adjacent ones thereof.
A generally U-shaped leaf spring 6 is inserted into each space 5 to urge the field magnetic poles peripherally outwardly so that the magnetic poles 4 are in pressure-contact with the inner peripheral surface of the cylindrical yoke 1 to thereby fix the magnetic poles 4 to the cylindrical yoke 1.
The leaf spring 6 has an axial end portion protruding from the space 5 and side wall portions 6a and 6b of the end portion are opened slightly as shown in FIG. 2 to restrict axial movement of the field magnetic poles 4.
In the structure shown in FIG. 1, four field magnetic poles 4 are used to provide four spaces 5 in two of which through-bolts 7 are provided axially. An armature 8 is rotatably supported within a region defined by the field magnetic poles 4.
In such structure as mentioned above, when the thickness of the field magnetic pole 4 is small and hence the height of the side walls of the spring is small, the stability of the spring in the space is low necessarily, causing an assembling operation of the motor to become difficult, or, when an outer diameter of the cylindrical yoke 1 is small and it is impossible to make the pitch of the through-bolt smaller, the space for the through-bolt becomes insufficient.